Compton Cameras have been used in the past, mainly in the gamma ray astrophysics field, to determine the energy and position in the sky of high energy celestial gamma ray emitters (Schönfelder, V. et al., “Instrument description and performance of the imaging gamma ray telescope COMPTEL aboard the Compton Gamma-Ray Observatory.” The Astrophysical Journal Supplement Series, 88: 657-892, 1993, Boggs, S. et al., “Overview of the nuclear Compton telescope.” New Astronomy Reviews, 48:251-256, 2004). More recently these kind of devices have been proposed for Nuclear Medicine imaging diagnostic purposes as they allow to reconstruct gamma ray emitting radioisotope distributions (Kahora, R. et al., “Advanced Compton camera system for nuclear medicine: Prototype system study.” Nuclear Science Symposium Conference Record, 2008, Harkness, L. J. et al., “Semiconductor detectors for Compton imaging in nuclear medicine”, 2012 JINST 7 C01004.). Compton cameras based on Si or CdTe have been recently proposed (Studen, A. et al., “First coincidences in pre-clinical Compton camera prototype for medical imaging.” Nucl. Instr. & Meth. 531 (2004) 258-264, Takeda, S. et al., “Applications and Imaging Techniques of a Si/CdTe Compton Gamma-Ray Camera.” Physics Procedia 37 (2012) 859-868.) due to the excellent energy resolution of semiconductor technology. However, semiconductor time resolution is very poor and does not allow for a sequential development of the whole gamma ray interactions.
All these devices, including the present invention, are based on the determination of the impact position of gamma ray interactions due to Compton scattering (FIG. 1). Compton scattering dominates the gamma ray detection process for energies between 150 keV and 5 MeV.
However, the most important limitation of current Compton cameras is that the timing resolution of the current systems does not allow one to determine the temporary order of the detected interactions on the different layers, of which the modules of these cameras are made. In fact, one of the major challenges of analyzing data from a combined Compton camera is the reconstruction of the parameters of each original gamma ray from the measured data, which consist only of several energy and position measurements.
For the complex task of Compton temporal sequence reconstruction, the detailed description of a dedicated multidimensional event data space naturally leads to a discussion of possible event quality selection criteria and their applicability to different event types, thus being very demanding from the computing point of view, while still producing low quality images. Recently Compton cameras with recoil electron tracking capabilities have been proposed (Boggs, S. et al., “Report on the Advanced Compton Telescope vision mission study. ‘Technical report”, NASA, 2005.), allowing the incident gamma ray direction to be confined inside a reduced cone arc region. However, these newly developed Compton cameras still lack the necessary timing accuracy information for efficiently determine the Compton temporal sequence, in U.S. Pat. No. 4,124,804 A “Compton scatter scintillation camera system” by S. Mirell, a method and apparatus for producing tomographic or cross-sectional radiographic images, from which the radiation is substantially confined to a single plane, and a conventional scintillation camera located to detect gamma radiation scattered from the object is presented, in Patent US 20140110592 A1 “Compton camera detector systems for novel integrated compton-Pet and CT-compton-Pet radiation imaging” by R. S. Nelson and W. B. Nelson, a novel Compton camera detector designs and systems for enhanced radiographic imaging with integrated detector systems which incorporate Compton and nuclear medicine imaging, PET imaging and x-ray CT imaging capabilities is described.
In U.S. Pat. No. 7,573,039 B2 “Compton camera configuration and imaging method” by B. D. Smith an approach for the selection of Compton camera shapes, configurations, positions, orientations, trajectory paths, and detector element sets is provided for collecting data for analysis using the surface integral and integral-of-line-integral methods of reconstruction Compton data. In U.S. Pat. No. 8,384,036 B2 “Positron emission tomography (PET) imaging using scattered and unscattered photons” by M. Conti TOF (time-of-flight) difference is obtained between the two gamma rays produced after positron annihilation. However, only those coincidence events where a full-energy gamma ray is detected at a first detector and a partial-energy scattered gamma ray is detected at a second detector is considered in that Patent. In U.S. Pat. No. 8,785,864 B2 “Organic-scintillator Compton gamma ray telescope” by Kenneth N. Ricci et al. an apparatus and methods for imaging sources of gamma rays with a large area are described, and comparatively low-cost Compton telescope is claimed, in U.S. Pat. No. 8,809,791 B2 “Continuous time-of-flight scatter simulation method” by P. Olivier and P. Khurd a method for correcting PET imaging data by simulation of time-of-flight scatter is presented.
However, none of the designs presented in these patents allow one to determine the complete timing sequence of the detected interactions produced by a single incident gamma ray.
The inability of current Nuclear Medicine devices to include Compton scattered events without degrading image quality is the most serious handicap to increase sensitivity in commercial scanners (PET and SPECT). This is due to the fact that current designs do not allow one to properly determine the order of the detected interactions produced by a single incident gamma ray within the gamma ray detector volume.
When dealing with PET scanners, the current technology focuses in detecting photoelectric events since the position of the first interaction in Compton events is unknown, and also it is not possible to distinguish Compton events in the detector from Compton events occurring inside the body. Therefore, events that lie outside the photoelectric peak are rejected because they produce noise and blurring in the image. However, these could amount to more than 50% of the events. Since PET works in coincidence mode, the probability for detecting two photoelectric events is less than 25% (of the order of 20%). Therefore, a factor 5 in sensitivity could be gained if Compton events are recovered.
PET Image quality is limited due to several factors including Compton scattered events inside the human body. Compton scattered events inside the body with the current technology are rejected through energy window around the photoelectric peak. The contribution of scattered events inside the photoelectric peak coming from scatter at the crystal or at the human body can currently only be estimated and corrected. The proposed invention will significantly improve image quality by a more efficient elimination of random and scattered events. The timing resolution of the current invention will be used to directly reject random events. Moreover, Compton and photoelectric temporal sequence events detection will be used to further eliminate scattered events inside the human body and random events by analysis of the kinematics of the whole positron-electron annihilation event.